As methods for smelting nickel oxide ore called limonite or saprolite, a method of dry smelting that produces nickel matt using a flash smelting furnace, a method of dry smelting that produces ferronickel using a rotary kiln or moving hearth furnace, a method of wet smelting that produces a mix sulfide using an autoclave, etc. have been known.
Upon charging the nickel oxide ore to the smelting step, pre-processing is performed for pelletizing, making into a slurry, etc. the raw material ore. More specifically, upon pelletizing the nickel oxide ore, i.e. producing pellets, it is common to mix components other than this nickel oxide ore, e.g., binder and reducing agent, then further perform moisture adjustment, etc., followed by charging into agglomerate producing equipment to make a lump on the order of 10 to 30 mm, for example (indicated as pellet, briquette, etc.; hereinafter referred to simply as “pellet”).
Ferronickel is an alloy of iron (Fe) and nickel (Ni), and is made as a raw material of stainless steel mainly; however, if the smelting reaction (reduction reaction) of the aforementioned pellets advances ideally, since one ferronickel grain is obtained for one of these pellets, it is possible for a comparatively large ferronickel grain to be obtained.
When considering the efficiency of recovering ferronickel grains from a reducing furnace after the reduction reaction, the grain size is important, and if the ferronickel grain splits in the course of the reduction reaction, not only will handling become difficult, but time and labor will be required in recovery, and depending on the case, a novel recovery apparatus becomes necessary; therefore, it is very disadvantageous in terms of cost.
For example, Patent Document 1 discloses technology of adjusting excess carbon content of the mixture in a mixing step to make a mixture by mixing raw materials including nickel oxide and iron oxide with carbonaceous reducing agent, as a pre-treatment method upon producing ferronickel using a moving hearth furnace.
However, upon producing pellets in the aforementioned way, in the case of nickel oxide ore being a raw material, if producing ferronickel, which is an iron-nickel alloy, by adjusting the raw material components other than nickel oxide ore in order to make so that the smelting reaction progresses effectively, the size of the obtained ferronickel grains will become smaller at the moment when the smelting reaction ends.
If the size of the obtained ferronickel grain becomes smaller, there are problems in that this ferronickel is far smaller than the size of the pellets with a diameter on the order of 10 mm to 30 mm, and split to no more than several millimeters; therefore, handling upon recovering from the reducing furnace is very difficult, and the recovery rate declines.
In other words, in a smelting method for producing ferronickel, which is an iron-nickel alloy, from nickel oxide ore, it is preferable to satisfy both conditions of: (1) the smelting reaction progressing effectively; and (2) suppressing the obtained ferronickel from splitting into small grains; however, with the conventional smelting technology, it is not possible to adequately satisfy the condition (2) in particular, and thus brings about a decline in recovery rate.
Patent Document 1; Japanese Unexamined Patent Application, Publication No. 2004-156140